The present invention relates to an apparatus and method for severing a mooring at a submerged location in response to a remote command.
It is often desired to temporarily moor vessels such as drilling ships, floating offshore drilling platforms, tankers, barges, buoys and the like in great depths of water. Such vessels are held in a substantially stationary position on the surface of the water by a length of anchor chain, cable or line attached to an anchor. In mooring in these great depths, particularly in areas of the world disposed toward the emergence of severe storms with little or no warning, it is desirable to have reliable, quickly releasable mooring systems. Typical emergency situations which require rapid movement of offshore drilling ships and platforms, in addition to the aforementioned severe weather, include well blowouts and drifting icebergs.
In the past, most mooring release mechanisms have been cumbersome to install and retrieve, unduly expensive to purchase and have required regular periodic maintenance at frequent intervals. For example, a prior art hydraulic anchor chain release mechanism which is installed beneath the anchor and anchor chain on the ocean floor is approximately 103 inches in length, 24 inches in diameter and weighs 4,400 pounds on land.
A prior art explosive cutting device for use on anchor chains, which merely comprises two shaped charges installed on an anchor chain link and detonated by explosive detonating cord, was found to be unsatisfactory in service as it required the shaped charges to extend one foot or more on each side of the anchor chain link in order to assure completer severance thereof upon detonation.
Other prior art explosive cutting devices are disclosed in U.S. Pat. No. 4,148,257, issued on Apr. 10, 1979 and assigned to Halliburton Company, Duncan, Okla. These devices generally comprise an upper member and a lower member, each having a linear shaped explosive charge therein, the members being clamped about an anchor chain link or other mooring element by a suitable fastening means, and explosive charge detonating means for each of the charges. In comparison to other prior art release mechanisms or cutters, the devices of U.S. Pat. No. 4,148,257 are relatively inexpensive, easily installed and requires relatively little maintenance. However, they are not reusable, and present several major problems in installation and operation. If the explosive cutting devices are to be installed above the water level, detonation thereof includes the ever-present possibility of flying debris which can damage the vessel or personnel unless shielding is provided. If the explosive cutting device is installed underwater but close to the hull of the vessel, the shock wave generated by shaped charge detonation may severly damage the hull. To avoid the possibility of damage, shielding is again required. Furthermore, in either of the two locations proximate the vessel one is losing most of the mooring when severance occurs. However, if the explosive cutting devices are placed further away from the vessel, the wires for detonation initiation become unwieldly to accommodate and present grave reliability and inspection problems. A remote (unwired) detonation system is impractical for these devices, as the shock wave from the explosion would destroy the required electrical components, the use of this is economically unfeasible for "one shot" use.
A prior art incendiary cutting torch for underwater use is disclosed in U.S. Pat. No. 3,713,636, issued on Jan. 30, 1973 and assigned to the United States of America. A nozzle configuration is shown which is suitable for cutting cables, chains or the like. However, no control system is disclosed which would permit remote commanded ignition of the torch. Furthermore, there is no suggestion as to how the torch would be disposed to cut a chain link to sever a mooring without the presence of an operator to move the torch as cutting proceeded.